Microcapsules with reduced shell wall permeability

ABSTRACT

Incorporation of inorganic colloidal particles into an encapsulation dispersion results in microcapsules having polymer shell walls further comprising colloidal inorganic particles. Capsules prepared in this manner have been found to be smaller in size, have a narrower size distribution, and exhibit decreased shell wall permeability. Capsules prepared using these colloidal particle dispersions are particularly useful in carbonless imaging constructions such as those containing a fill solution of a color precursor in a hydrophobic solvent and form deeply colored images when combined with a color developer.

This is a continuation of application Ser. No. 07/782,407 filed Oct. 25,1991, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to the preparation and use of microcapsuleshaving colloidal inorganic particles incorporated in the shell wall ofthe microcapsules. The colloidal particles are generally less than about0.03 microns (μm) in diameter and the shell may be formed by in-situ orinterfacial encapsulation techniques.

2. Description of Related Art

Technology has been available for many years to effectively providemicrocapsules with liquid oleophilic ingredients and many methods ofpreparing capsules have been developed. Most methods of encapsulationrequire two phases and make use of a dispersion or emulsion of one phasein another. Usually, the phases are a polar phase and a non-polar phase.Although in principle two immiscible organic phases could be used, inpractice there is generally an aqueous (polar) phase and an oilcontaining organic (non-polar) phase. Most commonly, the fill materialis the material to be encapsulated and is contained in the organicphase. Two methods of encapsulation that have achieved commercialutility are referred to as in-situ polymerization and interfacialpolymerization.

Matson, U.S. Pat. No. 3,516,941, discloses in-situ polymerizationreactions in which the material to be encapsulated is dissolved in anorganic, hydrophobic oil phase which is dispersed in an aqueous phase.The aqueous phase has dissolved resin precursors, particularlyaminoplast resin precursors, which upon polymerization will form thewall of the microcapsule. A dispersion of fine oil droplets is preparedusing high shear agitation. The degree of shear has a major effect onthe droplet size and may serve to keep the capsule size small. Additionof an acid catalyst initiates the polycondensation of the aminoplastprecursors within the aqueous phase, resulting in the formation of anaminoplast polymer which is insoluble in both phases. As thepolymerization advances, the aminoplast polymer separates from theaqueous phase and deposits on the surface of the dispersed droplets ofthe oil phase to form a capsule shell at the interface of the twophases, thus encapsulating the fill materials. This process produces themicrocapsules. Polymerizations that involve amines and aldehydes, suchas those described herein, are also known as aminoplast encapsulations.Urea-formaldehyde (UF), urea-resorcinol-formaldehyde (URF),urea-melamine-formaldehyde (UMF), and melamine-formaldehyde (MF),capsule formations proceed in this manner.

In interfacial polymerization, the materials to form the capsule wallare in separate phases, one in an aqueous phase and the other in a fillphase. Polymerization occurs at the phase boundary. Thus, a polymericcapsule shell wall forms at the interface of the two phases therebyencapsulating the fill materials. Wall formation of polyester,polyamide, and polyurea capsules proceeds via interfacialpolymerization.

The size distribution (volumetric diameter) of microcapsules is acritical parameter. There are numerous applications, such as incarbonless paper, where the volumetric diameter of the microcapsule mustbe within a specified range. Carbonless paper is widely used in theforms industry in preparing business forms. Typically, sheets ofcarbonless paper are printed upon to create a form, which is thencollated with other similarly printed upon sheets to create a form-setsuch that marking (as, for example, with a pen, pencil or typewriterkey) on the top sheet will provide the required number of duplicates.Traditionally, these carbonless paper forms have been printed byconventional printing techniques, such as offset lithography, etc. Withthe advent of high-speed electrophotographic copiers having dependable,high capacity collating systems, such copiers have been used to print oncarbonless paper. Such attempts have encountered problems becausecarbonless papers having microcapsules coated thereon are subject topremature rupture of the capsules when subjected to pressure, and highspeed copiers typically apply pressure to the sheets in various areaswithin the machine operation. The paper feed assembly station, the tonertransfer station, and the heat/pressure fuser station are examples ofplaces where sufficient pressure to rupture capsules can occur. Suchrupture leads to machine contamination as well as smudges and areas ofcolor development on the final, collated form-sets.

One approach to preparing carbonless papers for use inelectrophotographic copiers has been to use small capsules with a narrowsize distribution (see Kraft, U.S. Pat. No. 4,906,605). In general,small capsules are more resistant to accidental rupture than largercapsules. The narrow size distribution is necessary to insure that nolarge microcapsules are present that might rupture upon handling.Further, if the microcapsules protrude too far from the plane of thepaper, the microcapsules might be stripped off the paper or broken.Ideally, a 50% volumetric diameter of less than about 12 micrometers isdesired.

Suspending aids are commonly used in microencapsulation for formingsmall, unagglomerated capsules and have been found to give someadvantage in capsule manufacture. These suspending aids have beenorganic based materials such as polyvinyl alcohol andcarboxymethylcellulose. However, some of these aids interfere in the UFencapsulation process. For example, carboxymethylcellulose is capable ofproviding small capsules in urea-formaldehyde encapsulation. However,this can occur at the expense of wall permeability thereby making itpossible for the capsule contents to leak out or for undesirablesubstances to diffuse into the capsule.

Attempts to use water soluble polymers to control droplet size andsubsequent capsule size were carried out by Sinclair, U.S. Pat. No.4,396,670. Sinclair used water soluble polymers such asacrylamide-acrylic acid copolymers, anionic starch solutions, and sodiumalginate in the aqueous phase during encapsulations employingmelamine-formaldehyde. These water soluble polymers stabilize thedispersion of the oil phase with respect to the precondensate andinhibit droplet coalescence, thus controlling droplet size as well asstabilizing the dispersion. The water soluble polymer also reacts withthe melamine/formaldehyde precondensate to form the capsule shell wall.

Fukuo, U.S. Pat. No. 4,753,759, used an acrylic acid-methacrylic acid oran acrylic acid-itaconic acid copolymer in the aqueous phase to controlthe manufacture of capsules with shells of urea-formaldehyde,melamine-formaldehyde, or urea-melamine-formaldehyde polymers.

Solubilized inorganic materials have been used to modify the surface ofparticles to be encapsulated. Ugro, U.S. Pat. No. 4,879,175,encapsulated inorganic pigment particles in microcapsules prepared byin-situ polymerization (such as aminoplast polymerization), interfacialpolymerization, and coacervation. Because the pigment particles wereinsoluble in both the oil and water phases, Ugro used surface modifyingagents to control the relative wettability of the solids by the organicand aqueous phases. Surface modifying agents such as titanates andsilanes were used to modify the surface of the pigment, render itoleophilic, and thus encapsulable in the capsule fill (oil phase).Control of the relative wettability enabled the deposition of smooth,relatively fault free shells and could be used to control the locationof the pigments within the microcapsule structure. Pigments such asmetal oxides, carbon black, phthalocyanines, and particularly oil andwater insoluble cosmetic colorants were successfully encapsulated bythis method.

Terada, et al., U.S. Pat. No. 4,450,221, prepared magnetic tonerscomprising lyophilic magnetic particles and a resin surrounded by aresin wall to form microcapsules. Treatment with a titanate or silanecoupling agent was reported to uniformly disperse the particles in thebinder resin, firmly bond the magnetic particle to the resin, and renderthe surface of the magnetic particles lyophilic. Colorants such aspigments or dyes may be included in the wall forming resin or the toner.

Colloidal materials for forming a stable dispersion of unsaturated andsaturated oils in an aqueous phase are commonly used in suspensionpolymerization and mineral beneficiation. In suspension polymerization,monosize polymeric beads of approximately 5 μm diameter can be made.See, for example R. M. Wiley J. Colloid Sci. 1954, 9, 427. Wiley dealtwith the limited coalescence of oil droplets of coarse oil in wateremulsions, with a focus on bead size control in suspensionpolymerization. Wiley's findings were that the nature of the oil phasewas of minor importance provided it did not contain surface activegroups or impurities, and that the limiting size of the oil droplets isdirectly proportional to the product of oil phase volume and colloidparticle size, and inversely proportional to the weight of colloidemployed.

Colloidal silica particles have been used as a capsule fill material.U.S. Pat. No. 3,954,678 and U.S. Pat. No. 3,954,666 disclosedsemipermeable microcapsules containing catalysts and ferromagneticmaterials as well as colloidal and non-colloidal silica for use as anadsorbent for catalysts or as a chromatographic phase. Encapsulationswere carried out by interfacial polymerization and examples usingbis(acid chlorides) and diamines were detailed.

Colloidal silica particles have also been used as a capsule wallmaterial. Ohno, U.S. Pat. No. 4,579,779, employed silica as the soleshell wall material to encapsulate organic liquids. No polymers wereused in conjunction with the silica. The silica served to control thevolatilization and release of the encapsulated organic liquid.

None of the above cited references disclose the incorporation ofcolloidal inorganic materials into polymeric capsule walls. None of theabove work uses inorganic colloidal particles to control the dropletsize of the dispersed oil phase in which the continuous (aqueous) phaseis continually changing due to a polymerization reaction such as isoccurring during an encapsulation. In addition, none of the above workdiscloses the use of colloidal inorganic particles to control capsuleproperties in systems such as in carbonless paper. There exists a needfor microcapsules with controlled size, narrow size distribution, andlimited wall permeability.

SUMMARY OF THE INVENTION

This invention describes microcapsules comprising an oleophilic phaseretained within a synthetic polymer shell and a process for making thesame. The shell further comprises colloidal inorganic particles. Themicrocapsules have a 50% volumetric diameter ranging between about 3 to12 micrometers and are produced by a process comprising the steps of:

(a) dispersing and maintaining an oleophilic fill material as discretedroplets in an aqueous water-soluble pre-polymer solution comprisingcolloidal size inorganic particles, the fill material being inerttowards the pre-polymer and subsequent polymerization products and thecolloidal particles having a surface energy such that the particlesselectively position themselves at an interface between the oleophilicfill material and the aqueous solution, and

(b) polymerizing the pre-polymer solution while maintaining the fillmaterial as discrete droplets, whereupon there is produced an aqueousslurry of microcapsules having shell walls with the colloidal particlesincorporated in the shell wall.

Capsules prepared by this process can be used to encapsulatecolor-formers such as those used in carbonless copy-papers. Theencapsulated color-former may then be coated on paper and used in acarbonless paper application.

DETAILED DESCRIPTION OF THE INVENTION

Encapsulation as a means of separating reactive species to preventpremature reaction in certain commercially important products is wellknown. In one such product, the wall of the capsule is an aminoplastcondensation polymer and in another the wall is an interfacialcondensation polymer. We have discovered that the incorporation ofcolloidal inorganic particles in capsule walls controls capsule size andreduces permeability. To this end, the colloids must be present in theencapsulation media during the formation of the shell around theoleophilic phase.

The colloids must remain stable in the environment of the encapsulationprocess and must not interfere with the formation of the wall.Aminoplast condensation is normally carried out at very low pH whilesome interfacial polymerizations end at a pH over 8. The colloidalparticles become an integral part of the capsule shell during theencapsulation, modifying the properties of the shell and helping controlthe capsule size. The particles help stabilize the oleophilic phase inthe water phase at a controlled oil droplet size before and during thewall formation in the encapsulation process. This requires that thecolloid chosen for use be stable under the conditions met during anencapsulation process. For example, a colloid useful in aminoplastencapsulation would need to be stable in the acidic environment of theUF encapsulation. Thus salts such as tricalcium phosphate would not besuitable as colloids in aminoplast capsule formation because they arenot stable in the low pH reaction conditions. Similarly, to be stable inan interfacial polymerization, the colloid would need to be stable atthe pH conditions found in these processes.

Not all inorganic colloids are suitable for the encapsulation process.In order to be useful in an encapsulation process, the colloid mustsatisfy several requirements and must have certain properties.

The colloids must have the right wetting properties. Without the rightwetting properties, the colloids would stay dispersed in the aqueousphase or enter into the oil phase. The colloidal particles must have asurface energy which promotes migration of the particles to theinterface of the aqueous phase and the oil phase. Particles of varioustypes, when dispersed in water immiscible oils, and then mixed ordispersed under high shear into a water phase to produce oil in waterdispersions, will show a variety of behaviors depending upon the surfacecharacteristics of the particles relative to the oil and water phases.In particular, particles which are wetted by the water phase andincompletely or poorly wetted by the oil phase will readily move fromthe oil phase to the water phase during this dispersion process.Attempts to incorporate such a particle into the wall will generally beunsuccessful and lead to a capsule wall containing few, if any,particles. Particles which are completely wetted by the oil phase andincompletely or poorly wetted by the water phase will tend to remain inthe interior of the oil phase droplets during the dispersion process.Particles of this type will generally result in microcapsules having theparticles in the oil phase of the microcapsule with relatively fewparticles being abstracted from the oil phase or immobilized in themicrocapsule shell wall. Finally, particles which are incompletelywetted by either the oil phase or by the water phase will be foundconcentrated at the oil/water interface during such a dispersionprocess. Microcapsules formed using this type of particle will givecapsules having the particles in the shell wall of the microcapsule. Byin the shell wall is meant that the particles are fixedly associatedwith the shell either by being wholly entrained in the shell wall orpartially entrained at the outside surface or the inside surface of theshell wall. All of these configurations may be collectively described ashaving a polymer shell further comprising colloidal inorganic particles.The ability to select colloids with specific wetting characteristicsrelative to the chosen oil and water phase compositions provides themeans to control encapsulatability of the particle dispersions and themeans to control the ultimate location of the majority of the particleswithin the microcapsule (i.e., either fixed at or within themicrocapsule shell wall or freely dispersed in the core oil phase).

The interfacial tension relationship has been expressed in the followingequations (see, R. W. M. Lai and D. W. Fuerstenau Society of MiningEngineers, AIME, Transactions, 1968, 247, 549).

If γ_(SO) >γ_(WO) +γ_(SW) the colloids will disperse in the aqueousphase.

If γ_(SW) >γ_(WO) +γ_(SO) the colloids will disperse in the oil phase.

If γ_(WO) >γ_(SO) +γ_(SW) or if none of three interfacial tensions isgreater than the sum of the other two, the colloids will move to theoil/water interface.

In the above equations, γ_(SO) is the interfacial tension of thecolloid-oil interface, γ_(WO) is the interfacial tension of thewater-oil interface, and γ_(SW) is the interfacial tension of thecolloid-water interface. From these expressions it is clear that aspecific colloid may function well with solvents used in oneencapsulation and not perform well with a different encapsulationwherein the solvents used have a different interfacial tension.

The problems associated with determining the interfacial tension ofcolloidal particle interfacing with water or oil severely limit theusefulness of this relationship. It is apparent that there must be somemeans for determining the suitability of a specific colloid for use in aspecific encapsulation.

For the most efficient colloidal coating on the oil droplets, thecolloids should be free of agglomeration. Ideally, the colloids shouldbe predispersed in an aqueous phase.

It is desirable for the colloidal dispersion to contain from about 20%to 40% solids. Lower solids content may lead to greater dilution of theencapsulation reaction and possibly slow the capsule wall formation.

It is also desirable that the colloid particles have a mean size lessthan about 0.03 μm, since the capsule shell walls are about 0.1 to 0.2μm thick and larger particles may protrude through both the innersurface and the outer surface of the capsule shell. It is desirable thatthe colloidal particles coat the oil droplets and stabilize themthroughout the encapsulation process. The amount of colloid needed thusdepends on the total surface area of the oil droplets. Small oildroplets have a higher surface area (per unit weight) than large oildroplets and therefore require more colloidal particles forstabilization.

Size control of the capsules is very important in carbonless paper asdiscussed above. Thus, the microcapsules prepared in accordance with thepresent invention are especially useful in the manufacture of carbonlesspapers for use in electrophotographic copiers and copier duplicators,because small microcapsules may be manufactured in a reproduciblemanner. Size control is thought to result when the colloidal particleshave a certain interfacial tension with respect to the phases presentsuch that the colloidal particles tend to coat the oil droplets in anaqueous solution during initial homogenization or stirring with a highshear mixer. The coating protects the oil droplets from coalescence butdoes not prevent the droplets from breaking into a smaller size duringmixing and become incorporated in the wall during the encapsulationprocess. It should also be noted that other known suspending agents anddispersants, may also be used with the inorganic colloidal particlesdescribed herein.

Test for Selection of Suitable Colloids.

Selection of a suitable colloid may be carried out by a relativelysimple procedure. The colloidal dispersion to be evaluated is added towater in a mixing jar. The contents of the jar are homogenized at highspeeds in a suitable homogenization apparatus under high shearconditions for a few minutes while the oleophilic mixture of fillsolution material to be encapsulated is added. A few drops of theresultant dispersion are then removed, placed on a slide and examinedunder a microscope to estimate droplet size, droplet size distributionrange, and stability of the dispersion. Additional colloid may then beadded, and the procedure repeated until further addition has no effecton droplet size, or until the droplets are of the size desired. The pHshould also be adjusted to reflect the pH range encountered during theencapsulation process. To be a suitable candidate for an encapsulation,the dispersion must appear to be stable, and the droplet size should besmall. Experiment 1 shows the results of testing various colloids.

As noted above, to be useful in the encapsulation process, the colloidmust remain stable in the environment of the encapsulation process andmust not interfere with the formation of the wall. For UFencapsulations, the capsules formed by incorporating the colloids in thecapsule shell wall must be stable in the highly acidic environmentencountered during the encapsulation. That is, the incorporation ofcolloidal particles into the capsule shell wall must not have adeleterious effect on the capsules. A simple method for testingcompatibility of the colloid in the capsule wall is to do leak ratestudies using, for example, a Thermal Gravimetric Analysis (TGA)instrument for measuring loss of a volatile fill component at anelevated temperature over several hours.

Carbonless Imaging Constructions

As noted above, the invention further includes pressure sensitiveimaging systems, i.e., carbonless impact marking papers for the transferof images, employing capsules prepared as described herein.

Generally, a carbonless paper construction comprises at least two sheetsof paper, each with one surface, or side, coated with one of the twoprimary reactants. The two sheets are generally referred to as a donorsheet and a receptor sheet. When the coated faces, or surfaces, of thetwo sheets come into contact under sufficient activating pressure sothat the reactants can interact, a reaction occurs and an image forms onthe receptor sheet.

Carbonless imaging constructions generally involve coating capsulescontaining one reactant (i.e., the color-former) on one substrate, andcoating the other reactant (i.e., the developer) on another, mating,substrate (such as a sheet of paper). Means for preventing the reactionof the two reactants until intended, (i.e., until activating pressure isapplied), is also provided. Preferably, a fill solution of thecolor-forming compounds in a hydrophobic solvent is encapsulated orcontained in microcapsules and coated on the back side of a sheet ofpaper. The sheet is then mated with a receptor sheet coated with areactant for the color-forming compound. Such reactants include but arenot limited to transition metal salts, acid salts, acids, phenolics, andmetal phenolates. The microcapsules serve the purpose of isolating thereactants from one another (i.e., preventing reaction) until such timeas pressure is applied to the paper for the purpose of creating animage.

Carbonless papers are available commercially from a number of sources,and the chemistry used therein is of two general types. In onecommercial product, the capsules on a first sheet (donor sheet) comprisedithiooxamide (DTO) derivatives as the color-forming ligand dissolved inan appropriate hydrophobic solvent within microcapsules and coated ontoa back side of a donor sheet in a suitable binder. The back side of thedonor sheet is referred to herein as a coated back (CB) sheet. A metalsalt, preferably a Ni⁺² salt, optionally in a suitable binder, is coatedonto a front side of a mating, or receptor sheet, herein referred to asa coated front (CF). sheet. The receptor sheet with the transition metalcoated thereon comprises the transition salts of organic or inorganicacids. Preferred transition metal salts are those of nickel, althoughsalts of copper, iron, and other transition metals may be used in someapplications. Preferred acids useful in forming the transition metalsalts are mono-carboxylic aliphatic acids containing about 6 to 20carbon atoms, such as rosinic acid, stearic acid, and 2-ethylhexanoicacid. Nickel 2-ethylhexanoate and nickel rosinate are particularlypreferred transition metal salts. The composition including thetransition metal salt may be coated on substrates by conventionalcoating techniques. The term "suitable binder" refers to a material,such as starch or latex, that allows for dispersion of the reactants ina coating on a substrate. As stated previously, in imaging, the twosheets are positioned such that the back side of the donor (CB) sheetfaces the metal salt coating on the front side of the receptor (CF)sheet.

In another type of carbonless paper, the image results from the reactionbetween an encapsulated leuco dye color-former and an acid developer.The capsules on the back side of the donor sheet comprise leuco dyecolor-formers such as crystal violet lactone,3,3-bis(1-ethyl-2-methylindolyl)-3-phthalide,3-N,N-diethylamino-7-N,N-dibenzylamino)fluoran, or benzoyl leuco methylene blue. The receptorsheet, containing the developer, comprises an acidic material such assheets coated with an acidic clay, a phenolic, or a similar reagent,optionally in a suitable binder, to convert the colorless precursor toits colored form.

This invention is useful for the preparation of capsules using either ofthese imaging chemistries.

In the present invention, a donor sheet is coated with a slurrycomprising microcapsules having a polymer shell further comprisinginorganic particles. The microcapsules are filled with a suitablecolor-forming compound, dissolved in a suitable fill solvent orsolvents, preferably a hydrophobic solvent such that the solution iswater-insoluble. In addition to the colloidal materials, the shell ofthe capsules are preferably a water-insoluble urea-formaldehyde productformed by acid-catalyzed polymerization of a urea-formaldehydeprecondensate, as shown in G. W. Matson, U.S. Pat. No. 3,516,846,incorporated herein by reference. The capsule slurry may also becombined with a binding agent, such as aqueous sodium alginate, starch,or latex.

When activating pressure is applied to the untreated surface of thedonor sheet, the capsules rupture (i.e., those capsules corresponding tothe pattern of applied pressure) and release the solution of theencapsulated color-former for transfer to the receptor sheet. Upontransfer, a reaction between the previously separated reactants occursand a color forms on the receptor sheet.

In many applications the uncoated surface of the donor (CB) sheetcomprises a form of some type and the activating pressure is generatedby means of a pen or other writing instrument used in filling out theform. Thus, "activating pressure" includes, but is not limited to,pressure applied by hand with a stylus or pressure applied by impactsuch as by a business machine key, a typewriter key or a computerprinter.

Substrates with one surface on which is coated the encapsulatedcolor-former, and a second, opposite surface on which is coated thedeveloper can be placed between the CF and CB sheets, in a constructioninvolving a plurality of substrates. Such sheets are generally referredto as a CFB sheets (i.e., Coated Front and Back sheets). Of course, eachside including ligand thereon should be placed such that the CF is inmutual contact with the CB. CFB sheets are typically used inconstructions requiring multiple sheets in a single pad.

EXPERIMENTAL EXAMPLES

The following examples are presented to illustrate the operation of theinvention and are not to be construed as limiting its scope.

Experiment 1

Evaluation of Suitable Colloids.

As noted above, selection of a suitable colloid may be tested by arelatively simple procedure. A colloidal dispersion of 7.5 g ofcolloidal silica available under the trade designation as Nalco 1034Acolloidal silica was added to 105.0 g of water in an 8 ounce jar. Nalco1034A is an aqueous acidic silica colloidal dispersion sold by NalcoChemical Company, Oak Brook, Ill. It has a concentration of 34% solidswith a mean particle size of 0.02 μm and a pH of 3.2. About 22.5 g of anoleophilic mixture of the capsule fill material was added and high speedstirring was begun. A Silverson homogenizer with a 0.75 inch mixing headat 2800 rpm was used. The oleophilic mixture of capsule fill materialcomprised 23.2 wt % of tributyl phosphate (TBP), 15.5 wt % of diethylphthalate (DEP), 49 wt % of cyclohexane, and 12.3 wt % ofN,N'-(dioctanoyloxyethyl)dithiooxamide (DOEDTO) color-former. Thehomogenization of the mixture proceeded for 2 min after the addition wascomplete. A sample of dispersion was removed and examined under amicroscope to estimate droplet size and stability of the dispersion. Thedroplets were observed to be spherical with no coalescence taking place.Droplet size was measured with a Coulter model TA-II size analyzerequipped with a 140 μm aperture tube. The median of the population andvolumetric distributions were 3.4 and 4.3 μm, respectively.

The Nalco 1034A colloid is thus suitable for testing in encapsulationssuch as in-situ polymerization to form, for example, urea-formaldehyde,urea-resorcinol-formaldehyde urea-melamine-formaldehyde, ormelamine-formaldehyde capsules, and for interfacial polymerization toform, for example, polyurea capsules. Other colloidal particles weretested according to the above procedure including Nyacol 50/20 colloidsand Cab-O-Sil colloids, both which had suitable results (available fromNyacol Products, Inc., Ashland, Mass. and Cabot Corporation, Tuscola,Ill., respectively.) Table 1 shows the results of testing variouscolloids. Dispursal, Nalco ISJ-614 colloids and Nyacol 100/20 colloidsall resulted in droplet sizes which exceeded the desired range of thisinvention. However, Nalco 1034A, Cab-O-Sil EH-5 and Nyacol 50/20colloids are all acceptable colloids, for the present invention. Theresultant dispersions were stable using these colloids and the dropletsizes were within the desired range of this invention.

                                      TABLE 1                                     __________________________________________________________________________    Evaluation of Colloidal Dispersions                                           for Stabilizing a Fill Containing                                             5% DOEDTO, 4% Pergascript Olive,                                              3.1% CAO-5, 41.3% DEP, 46.6% Cyclohexane                                                       Colloid Size                                                 Colloid Chemical Nature                                                                        (diameter)                                                                            Results                                              __________________________________________________________________________    Dispural                                                                              --AlO(OH)                                                                              0.055-0.085 μm                                                                     Fill droplets coalesced even after                                            adding more Dispural                                 Nalco ISJ-614                                                                         Alumina sol                                                                            2 μm Additional colloid was necessary.                                             Droplets were spherical and droplet                                           size ranged from 10 to 55 μm in                                            diameter. The dispersion appeared                                             to be stable, but particle size                                               range was broad.                                     Nyacol 100/20                                                                         Zirconium dioxide                                                                      0.1 μm                                                                             Additional colloid was necessary,                            sol              dispersion was stable. Droplet size                                           ranged from 2 to 44 μm                            Nalco 1034A                                                                           Silica sol                                                                             0.02 μm                                                                            Dispersion was stable, no additional                                          colloid was needed. Droplet size                                              ranged from 3 to 11 μm                            Cab-O-Sil EH-5                                                                        Fumed silica                                                                           0.007 μm                                                                           Dispersion was stable, additional                                             colloid was needed. Droplet size                                              ranged from 3 to 9 μm.                            Nyacol 50/20                                                                          Zirconium dioxide                                                                      0.05 μm                                                                            Dispersion was stable. Droplet                               sol              ranged from 2 to 10 μm.                           __________________________________________________________________________

Experiment 2

Urea-Formaldehyde Encapsulation using Colloidal Silica.

The encapsulation was based on the method described by Matson. (See, G.W. Matson, U.S. Pat. No. 3,516,941.) A precondensate solution wasprepared comprising 191.88 g of formalin (37% formaldehyde), 0.63 g oftriethanol amine, 71.85 g of urea, and, 327.93 g of water contained in abaffled one liter reactor equipped with a stirrer and water bath. Thetriethanol amine and urea were added first, followed by formalin. Themixture was heated to 71.1° C. and the reaction was maintained at 71.1°C. for 2.5 hours. The reaction mixture was then diluted with the waterand allowed to cool. The precondensate solution, with about 24% solids,was then ready for use in the encapsulation process.

The precondensate solution and fill were combined to make capsulesaccording to the following procedure. The temperature of the reactor wasset to 21.1° C., and 500 g of UF precondensate, 40 g of Nalco™ 1034Acolloidal silica, 30 g of NaCl, and 97 g of water were added and mixedto dissolve the salt. The mixture was homogenized with a Tekmar SD-45homogenizer equipped with a G456 head at 7200 rpm and 202 g of fillsolution was added and homogenized for 10 minutes. The fill solutioncomprised N,N'-(dioctanoyloxyethyl)dithiooxamide, 24.8 g (12.3%);diethyl phthalate, 31.3 g (15.5%); tributyl phosphate, 46.9 g (23.2%);and cyclohexane, 99.0 g (49.0%). The homogenizer was removed and thecontents were stirred with a three blade stirrer set 0.5 inch from thebottom of the reactor. The stirrer was set at a speed of 930 rpm. After5 minutes, 10% HCl was added over a 5 minute period to adjust the pH to2.85. After 12 minutes, 10% HCl was added over about a 12 minute perioduntil the pH dropped to 1.85. After one hour, the reactor temperaturewas raised to 60° C. and maintained at that temperature for 1.75 hr tocure the capsules. After curing, the slurry was neutralized with ammoniato pH 8 and cooled to room temperature and the particle sizedistribution was measured with a Coulter TA-II particle size analyzerequipped with a 140 μm aperture. The median population diameter was 5.1μm. The 50% volumetric diameter was 8.7 μm.

The capsule slurry (10 g) was added to 65 g of a 1.5% aqueous sodiumalginate solution. The mixture was applied to a coated paper using a barcoater with a 75 μm (3 mil) gap. The coating was allowed to dry at roomtemperature, and was found to image well as a CB sheet in a carbonlesspaper construction with a CF sheet coated with a nickel salt. (Thissheet was obtained from the Carbonless Products Department of MinnesotaMining and Manufacturing Company, St. Paul, Minn.)

Experiment 3

Urea-Formaldehyde Encapsulation Using Colloidal Zirconium Dioxide.

A urea-formaldehyde precondensate was prepared as shown in Experiment 2above. The UF precondensate (499.92 g) was aged overnight and was cloudyat pH 8.13. Sodium chloride (30 g) was added and the mixture was chargedin a 1 liter baffled reactor. Nyacol Zr50/20 colloidal solution (40 g)was added and the pH was adjusted to 7.0 by the addition of 10% sodiumhydroxide solution. Nyacol Zr50/20 colloidal solution is an acidstabilized 20% aqueous colloidal solution of zirconium oxide having amean particle size of 0.05 pm, and a pH of 3.0. The fill (202 g) wasadded over 5 min and the mixture was homogenized with a Tekmar SD-45homogenizer equipped with a G456 head at 7200 rpm for ten minutes. Thefill solution comprised N,N'-(dioctanoyloxyethyl)dithiooxamide, 10.1 g(5.0%); Pergascript Olive color former, 8.1 g (4.0%); CAO-5 antioxidant,6.3 g (3.1%); diethyl phthalate, 83.5 g (41.3%); and cyclohexane, 94.1 g(46.6%). Pergascript Olive is a color-former sold by Ciba-Geigy andCAO-5 is an anti-oxidant sold by Sherwin-Williams Corporation. Thedispersion had a particle size of about 2 to 15 μm as viewed with amicroscope. The reactor was placed in a water bath at 21.1° C. and athree blade stirrer set 0.5 inch from the bottom of the reactor was setat 930 rpm to stir the mixture. After 5 minutes 10% hydrochloric acidwas added dropwise over a 5 minute period to bring the pH to 3.02. Afteran additional 12 minutes, the pH was adjusted down to 1.85 by the slowaddition of 10% hydrochloric acid over an additional 12 minutes. Themixing was continued for 2 hours at 21.1° C., then the bath temperaturewas raised to 60° C. and mixing continued for 1.75 hours to cure thecapsules. The pH was raised to 8 by the addition of ammonium hydroxideto terminate the reaction.

Capsule size, determined by examination of the diluted capsule slurrywith a microscope, indicated that the capsules were between 3 and 22 μmin diameter. The walls were smooth with a spherical to oval shape.Coulter analysis showed a 50% volumetric diameter of 9.1 μm.

The capsule slurry (10 g) was added to 65 g of a 1.5% aqueous sodiumalginate solution. The mixture was applied to a coated paper using a barcoater with a 0.076 millimeter gap. The coating was allowed to dry atroom temperature, and was found to image well as a CB sheet in acarbonless paper construction with a CF sheet coated with a nickel salt.

Experiment 4

Urea-Formaldehyde Encapsulation with No Colloid Present.

The encapsulation in Example 2 was repeated, but without the colloidalparticles added. Upon neutralization with ammonia, evaluation of thecapsule size indicated a 50% volumetric diameter of 20.8 μm. Thevolumetric diameter exceeded the preferred range of 3-12 μm. A CB sheetwas prepared using sodium alginate solution as in Experiment 2 above andwas found to image well.

Experiment 5

Urea-Resorcinol-Formaldehyde Encapsulation Using Colloidal ZirconiumDioxide.

A one liter baffled reactor was charged with 518 g of water, 11.0 g ofurea, 1.1 g of resorcinol, and 40.0 g of Nyacol Zr50/20 colloidalsolution. A Tekmar SD-45 homogenizer equipped with a G456 head was usedfor making the dispersion. The homogenizer was set 1.27 cm from thebottom of the reactor and the mixture stirred at 7200 rpm. A fillsolution was added in the amount of 187.61 g and the mixture washomogenized for 10 min. The fill solution comprisedN,N'-(dioctanoyloxyethyl)dithiooxamide, 9.4 g (5.0%); Pergascript Olivecolor former, 7.5 g (4.0%); CAO-5 antioxidant, 5.8 g (3.1%); diethylphthalate, 77.5 g (41.3%); and cyclohexane, 87.4 g (46.6%). A sample ofthe dispersion was examined with a microscope and a droplet diameter of2 to 10 μm was observed. The dispersion appeared to be stable. The pH ofthe water phase was 1.93 after the Nyacol Zr50/20 was added to theurea-resorcinol solution. The pH was raised to 3.5 by the addition of10% sodium hydroxide solution. Some thickening of the dispersionoccurred but this was only temporary.

The reactor was placed in a water bath set at 50° C., the homogenizerwas replaced with a Cole Palmer 5.08 cm diameter three blade agitator(Cole Palmer Catalog No. N-0544-10) set 1.27 cm from the bottom of thereactor, and stirred at about 930 rpm, after which 27.6 ml of 37%formaldehyde solution was added. Stirring was continued for 2 hours, andthe pH dropped to 2.90 during the encapsulation. The temperature waslowered to 25° C., and 25 ml of 28% ammonium hydroxide solution wasadded to neutralize the slurry and bring the pH to 7.0.

The capsule walls were mostly smooth and the shape ranged from sphericalto oblate. Coulter analysis gave a 50% volumetric diameter of 4.5 μm,which is in the preferred range of 3-12 μm.

Experiment 6

Urea-Resorcinol-Formaldehyde Encapsulation Using Colloidal Silica.

A one liter baffled reactor was charged with 518 g of water, 11.0 g ofurea, 1.10 g of resorcinol, and 5.00 g of Cab-O-Sil™ EH-5. Cab-O-Sil™EH-5 has a mean particle size of 0.007 μm. The pH of the water phase was4.45 after the Cab-O-Sil™ silica was added to the urea-resorcinolsolution. The pH was lowered to 3.5 by the addition of 27% acetic acid,and 187.61 g of fill solution was added. The fill solution comprisedN,N'-(dioctanoyloxyethyl)dithiooxamide, 23.1 g (12.3%); diethylphthalate, 29.1 g (15.5%); tributyl phosphate, 43.5 g (23.2%); andcyclohexane, 91.9 g (49.0%).

After the addition of the fill, the mixture was homogenized at 7200 rpmfor 10 min using a Tekmar SD-45 homogenizer equipped with a G456 head.The droplets were 30 μm or less in diameter. The dispersion was stable,and the Coulter analysis showed a median droplet diameter of 11.25 μm.

An additional 5 g of Cab-O-Sil™ EH-5 silica was added and the mixturewas homogenized for an additional 10 minutes. The Coulter analysisshowed the median droplet diameter to be reduced to 10.86 μm. Thereactor contents had heated to 66° C. during the homogenization.

The reactor was placed in a water bath set at 50° C., the homogenizerwas replaced with a Cole Palmer 5.08 cm diameter three blade agitator(Cole Palmer Catalog No. N-0544-10) set 1.27 cm from the bottom of thereactor, and stirred at about 800 rpm, until the contents of the reactorhad equilibrated with the bath temperature; then 27.6 ml of 37%formaldehyde was added. Stirring was continued for 2 hours, thetemperature was lowered to 25° C., and 7 ml of 28% ammonium hydroxidewas added to neutralize the slurry and bring the pH to 7.3. The capsuleswere spherical and the walls were smooth. Data from the Coulter particlesize analyzer data showed the capsule size distribution had a 50%volumetric diameter of 10.3 μm, which is in the preferred range of 3-12μm.

A CB sheet coated with the capsules in sodium alginate in the manner ofExperiment 4 gave a good image with the CF sheet.

If the amount of Cab-O-Sil colloidal silica in the above experiment wasraised to 15 g, the aqueous phase became so viscous that much morevigorous agitation was necessary to achieve a uniform mixing.

Experiment 7

Interfacial Polymerization Encapsulation Using Colloidal Silica--Nalco1034A.

A one liter baffled reactor was charged with water, 550.00 g; Nalco1034A colloidal silica, 37.00 g; and fill, 180 g. The fill consisted ofN,N'-(dioctanoyloxyethyl)dithiooxamide, 17.6 g; diethyl phthalate 23.9g; tributyl phosphate 35.9 g; toluene, 75.6 g; and Mondur MRSisocyanate, 27.00 g. Mondur MRS is a polymethylene polyphenyl isocyanatemanufactured by Mobay Chemical Corporation. The temperature of thereactor was equilibrated at 21.1° C. and the reactor contents were mixedwith a Waring Blender blade set 1.27 cm off the reactor bottom and witha speed set to 2300 rpm. After 5 min of mixing, 180 ml of a 25% solutionof tetraethylene pentamine in water was added dropwise over one hour.Stirring was continued for an additional hour after which a sample waswithdrawn and the particle size was determined. The capsules producedhad a 50% volumetric diameter of 6.3 μm and a 95% volumetric diameter of14.3 μm or less.

The capsules were evaluated in a coated CB sheet, as in Experiment 2above, and were found to give good image density.

Experiment 8

Interfacial Polymerization Encapsulation Using ColloidalSilica--Cab-O-Sil™ EH-5.

A one liter baffled reactor was charged with water, 550.00 g; Cab-O-SilEH-5 colloidal silica, 10.94 g; and fill 180 g. The fill consisted ofN,N'-(dioctanoyloxyethyl)dithiooxamide, 17.6 g; diethyl phthalate 23.9g; tributyl phosphate 35.9 g; toluene 75.6 g; and Mondur MRS isocyanate,27.00 g. The temperature of the reactor was equilibrated at 18° C. andthe reactor contents were homogenized with the Tekmar SD-45 homogenizerwith the G 456 head. The speed was set at 7,200 rpm and homogenized forten minutes. The homogenizer was removed and the reactor was stirredwith a 6 flat blade agitator with speed set for 1150 rpm while 153 ml of25% tetraethylene pentamine was added dropwise. Mixing was continued forone hour, then a sample was removed for particle size analysis. Thecapsules produced had a 50% volumetric diameter of 5.4 μm and a 95%volumetric diameter of 9.3 μm or less.

If the amount of Cab-O-Sil colloidal silica in the above experiment wasraised to 15 g, the aqueous phase became so viscous that much morevigorous agitation was necessary to suspend the oil droplets.

The following experiments demonstrate the use and advantage of colloidalsilica in urea-melamine-formaldehyde (UMF) and urea-formaldehyde (UF)encapsulations.

Experiment 9

Urea-Melamine-Formaldehyde Encapsulation using Colloidal Silica.

A precondensate solution was prepared comprising 180.89 g of formalin(37% formaldehyde), 57.3 g of urea, 10.71 g of melamine, and 0.64 g ofpotassium tetraborate contained in a 1-L reactor equipped with a stirrerand water bath. The potassium tetraborate melamine, and urea were addedto the reactor followed by the formalin. The mixture was heated to 71.1°C. and was maintained at that temperature for 2.5 hours. The reactionmixture was then diluted with 285.81 g water and allowed to cool to roomtemperature and age overnight. The precondensate solution was then readyfor use in the encapsulation process.

The precondensate solution and fill were combined in a one liter reactorto make capsules according to the following procedure. The temperatureof the reactor was set to 21.1° C., and 535.33 g of the UMFprecondensate, 45.00 g of Nalco 1034A colloidal silica, 30.59 g of NaCl,and 77.93 g of water were added and mixed. Upon dissolution of the salt,the mixture was stirred for 5 minutes at 2300 rpm with a Waring blenderblade set 1.26 cm of the reactor bottom, and 192.66 g of fill solutionwas added. The fill solution comprisedN,N'-(dioctanoyloxyethyl)dithiooxamide, 10.5%;N,N'-dibenzyldithiooxamide, 1.50%; diethyl phthalate, 15.62%; tributylphosphate, 23.44%; and cyclohexane, 49.44%. After 5 minutes, 10% HCl wasadded over 5 minutes to adjust the pH to 3.00 and to catalyze the UMFpolymerization. After 12 minutes, an additional 10% HCl was added over12 minutes to adjust the pH to 1.85. The reaction was allowed to stirfor 1 hr at 21.1° C. The reactor temperature was raised to 60° C. andmaintained at that temperature for 1.75 hr to cure the capsules. Aftercuring, the slurry was neutralized with ammonia to pH 8, cooled to roomtemperature, filtered through a 500 μm screen, and stored.

Particle size, determined by evaluation with a Coulter TA-II particlesize analyzer equipped with a 140 μm aperture tube indicated a 50%volumetric diameter of 10.1 μm.

Experiment 10

Urea-Melamine-Formaldehyde Encapsulation without Colloidal Silica.

UMF capsules were prepared as described above but without colloidalsilica present in the encapsulation media. Capsules thus obtained werevery large in size and had a 50% volumetric diameter of 40.8 μm.

Experiment 11

Urea-Formaldehyde Encapsulation using Colloidal Silica.

A precondensate solution was prepared comprising 191.88 g of formalin(37% formaldehyde), 71.5 g of urea, and 0.63 g of potassium tetraborate,contained in a 1-L reactor equipped with a stirrer and water bath.Potassium tetraborate, urea, and formalin were added and the mixture washeated to 71.1° C. and the reaction was maintained at that temperaturefor 2.5 hours. Water, 327.93 g was then added, the reaction was allowedto cool to room temperature and aged overnight. The precondensatesolution was then ready for use in the encapsulation process.

The precondensate solution and fill were combined in a 1-L reactor tomake capsules according to the following procedure. The temperature ofthe reactor was set to 21.1° C., and 533.80 g of the UF precondensate,prepared above, 45.00 g of Nalco 1034A colloidal silica, 30.56 g ofNaCl, and 79.00 g of water were added and mixed. Upon dissolution of thesalt, the mixture was stirred for 5 minutes at 2300 rpm with a Waringblender blade set 1.26 cm off the reactor bottom, and 193.02 g of fillsolution was added. The fill solution comprisedN,N'-(dioctanoyloxyethyl)dithiooxamide, 10.5%;N,N'-dibenzyldithiooxamide, 1.50%; diethyl phthalate, 15.62%; tributylphosphate, 23.44%; and cyclohexane, 49.44%. After 5 minutes, 10% HCl wasadded over 5 minutes to adjust the pH to 3.00 and to catalyze the UMFpolymerization. After 12 minutes, an additional 10% HCl was added over12 minutes to adjust the pH to 1.85. The reaction was allowed to stirfor 1 hour at 21.1° C. The reactor temperature was raised to 60° C. andmaintained at that temperature for 1.75 hours to cure the capsules.After curing, the slurry was neutralized with ammonia to pH 8, cooled toroom temperature, filtered through a 500 μm screen, and stored.

Particle size, determined by evaluation with a Coulter TA-II particlesize analyzer equipped with a 140 μm aperture indicated a 50% volumetricdiameter of 7.8 μm.

The results of Experiments 2-11 are shown in the following Table 2.

                  TABLE 2                                                         ______________________________________                                        50% Volumetric Diameter of Capsules Prepared                                  Using Various Dispersing Aids                                                                                   50%                                                 Encapsulation             Volumetric                                  Example Type        Colloid       Diameter                                    ______________________________________                                        2       UF.sup.1    Nalco 1034A   8.7 μm                                   3       UF.sup.1    NyacolZr50/20 9.1 μm                                   4       UF.sup.1    None          20.8 μm                                  5       URF.sup.2   Nyacol Zr50/20                                                                              4.5 μm                                   6       URF.sup.2   Cab-O-Sil EH-5                                                                              10.3 μm                                  7       IF.sup.3    Nalco 1034A   6.3 μm                                   8       IF.sup.3    Cab-O-Sil EH-5                                                                              5.4 μm                                   9       UMF.sup.4   Nalco 1034A   10.1 μm                                  10      UMF.sup.4   None          40.8 μm                                  11      UF.sup.1    Nalco 1034A   7.8 μm                                   ______________________________________                                         UF.sup.1 = ureaformaldehyde shell                                             URF.sup.2 = urearesorcinol-formaldehyde shell                                 IF.sup.3 = interfacial polymerization polyurea shell                          UMF.sup.4 = ureamelamine-formaldehyde shell                              

Table 2 illustrates the 50% volumetric diameters of capsules notemploying colloids exceeds 12 μm, which is above the preferred range of3-12 micrometers. Thus, the capsules not employing colloids exceeds theupper useful limit of the present invention.

The following experiments demonstrate the use of colloidal particles inthe formation of capsules containing acid-tripped leuco dyecolor-formers.

Experiment 12

Urea-Formaldehyde Encapsulation using Colloidal Silica.

This encapsulation was based on that described by Matson (see, G. W.Matson, U.S. Pat. No. 3,156,941. A precondensate solution and fill werecombined as in Experiment 2 above, to make capsules according to thefollowing procedure. The temperature of the reactor was set to 21.1° C.,and 2,249.67 g of UF precondensate, 467.01 g of Nalco 1034A colloidalsilica, 197.64 g of NaCl, and 1,712.47 g of water were added and mixedto dissolve the salts. The mixture was homogenized with a 7.00 cmdiameter bar turbine set 5 cm off the bottom of the reactor at 3000 rpmand 1,733.51 g of fill solution was added and homogenized for 10 min.The fill solution comprised Pergascript Orange I-5R color former, 8.67g, (0.50%); Pergascript Red I-6B color former, 5.20 g (0.30%);Pergascript Blue I-2R color former, 6.93 g (0.40%); Pergascript GreenI-2GN color former, 22.54 g (1.30%); Pergascript Black I-R, 43.34 g(2.50%); diethyl phthalate, 741.08 g (42.75%); and cyclohexane 905.76 g(52.25 g). After 5 minutes, 10% HCl was added to adjust the pH to 2.85.After 22 minutes, 10% HCl was added until the pH dropped to 2.43. After28 minutes, 10% HCl was added to adjust the pH to 2.07. After 34minutes, 10% HCl was added to adjust the pH to 1.70. After 1.75 hour,the reactor temperature was raised to 60° C. and maintained at thattemperature for 0.75 hr to cure the capsules. After curing, the slurrywas neutralized with 30% ammonium hydroxide solution and cooled to roomtemperature, filtered through a 500 pm mesh screen, and particle sizeevaluated with a Coulter TA-II particle size analyzer equipped with a140 μm aperture tube. The 50% volumetric size was 5.51 μm. A CB sheetwas prepared using sodium alginate solution as in Experiment 2 above andwas found to image well in a carbonless paper construction with a CFsheet coated with an acid developer.

Experiment 13

The encapsulation described in Experiment 12 was repeated using Nalco1042 colloidal silica to afford capsules having a 50% volumetricdiameter of 6.28 μm. Nalco 1042 differs from Nalco 1034A colloidalsilica in having a turbidity (Hach) NTU value of 140. Nalco 1034Acolloidal silica has a turbidity (Hach) value NTU of 190.

Experiment 14

Effect of Colloid Incorporation on Wall Permeability.

A one liter baffled reactor was charged with water, 218.00 g; Nalco1034A colloidal silica, 15.00 g; and fill, 137 g. The fill consisted ofReldan™ insecticide, 132 g of a 66.1% solution of active ingredient inan organic solvent; Igepalco-630, 1.00 g; and Mondur MRS isocyanate,4.00 g. (Reldan™ is an insecticide manufactured by Dow Chemical Company;and Igepal Co-630 is a nonionic surfactant manufactured by GAFCorporation.) The temperature of the reactor was 21.1° C. and thereactor contents were stirred with a Waring Blender blade set 2.54 cmfrom the bottom of the reactor. The speed was maintained at 2000 rpm.Tetraethylene pentaamine, 28 gm of a 10.7% solution in water was addeddropwise. Stirring was maintained. A sample was withdrawn and theparticle size was determined. The 50% volumetric diameter was 15.4 μm.

As a comparative example, the above reaction was run in an identicalmanner except that the Nalco 1034A colloidal silica was replaced with 15g of water. The 50% volumetric diameter was 15.7 μm.

Two grams of each capsule slurry were placed in flasks and stirred witha 3 propeller blade agitator. A solution of 691.6 g (700 ml) ofpropylene glycol containing 15 wt % ethanol was added to each flask.After a given time, a 1-2 ml sample of liquid was removed and filteredthrough a 0.2 μm disc filter into a vial for analysis of percent Reldanextracted. The samples were analyzed by gas chromatography on aHewlett-Packard HP 5890 gas chromatograph and HP 3303A integrator,equipped with a flame ionization detector and employing helium as thecarrier gas. A fused silica DB-5 capillary column (15 m×0.246 mm) with a0.25 μm film thickness was employed. The temperature, was maintained at215° C.

The results of the extractions, shown below in Table 3, demonstrate thatincorporation of colloidal particles into capsule walls prepared byinterfacial polymerization results in decreased permeability of thecapsule walls.

                  TABLE 3                                                         ______________________________________                                        Permeability of Reldan ™ from Capsules                                     Prepared With and Without Colloidal                                           Particles in the Shell Wall.                                                            % Reldan ™ Remaining in Capsules                                             with Particles                                                                           without Particles                                      Time (hr)   in shell.sup.1                                                                           in shell                                               ______________________________________                                        0.25        100.0      94.9                                                   0.50        95.7       94.9                                                   1.00        96.1       92.3                                                   1.50        94.8       90.3                                                   2.00        95.3       85.6                                                   3.00        94.6       80.2                                                   5.50        92.8       48.2                                                   7.00        91.4       46.5                                                   8.25        90.4       36.2                                                   24.00       81.2       17.1                                                   29.00       77.6        8.0                                                   48.00       67.6        6.9                                                   ______________________________________                                         .sup.1 Colloidal dispersion used was Nalco 1034A                         

Experiment 15

Effect of Colloidal Particles Incorporation on Wall VolatilePermeability.

Capsules from Experiments 2 and 4 were filtered, washed and dried. Thedried capsules were tested by thermal gravimetric analysis to determinethe rate of weight loss at 100° C. of the volatile component of the fill(cyclohexane). The capsule weight loss is shown in Table 4. In theinitial ten minutes there is a fast weight loss attributed to waterescaping that was once retained by the UF polymer. Loss in weight afterthis period is due to cyclohexane escaping through the capsule wall.

                  TABLE 4                                                         ______________________________________                                        Thermogravimetric Analysis of Capsules With                                   and Without Colloidal Particles in the Shell                                               (% weight loss)                                                                          (% weight loss)                                                    with particles                                                                           without particles                                     Time (min)   in shell   in shell                                              ______________________________________                                         10          0.072      0.040                                                  90          0.123      0.310                                                 170          0.180      0.452                                                 250          0.185      0.539                                                 330          0.211      0.600                                                 410          0.261      0.672                                                 490          0.283      0.734                                                 570          0.332      0.775                                                 650          0.334      0.816                                                 730          0.360      0.860                                                 ______________________________________                                    

The weight loss for capsules made with shells containing the colloidalsilica was much lower than the capsules made without the colloidalsilica. It is preferred to have a permeability of less than about 0.250percent weight loss after 250 minutes.

Experiment 16

Effect of Colloidal Particles on Acid Resistance of Capsules.

Forty grams of UF capsules prepared according to Experiment 2 above,were placed in a separatory funnel. Fifty grams of water was added,followed by 100 ml of concentrated hydrochloric acid (37%) resulting inabout a 24% hydrochloric acid solution. The funnel was stoppered and thecontents vigorously shaken and the layers allowed to separate. After 1hr the contents of the funnel were inspected. The capsule phase (orupper layer) had only a small amount of free fill. A sample of the upperlayer was inspected under a microscope and found to contain mostlyundamaged capsules. Thus, the shell walls remain structurally intactafter exposure to an acid for at least 60 minutes.

Replacement of the hydrochloric acid with sulfuric acid (98%) gaveessentially the same results.

Using 40 g of capsules prepared according to Experiment 4, that is,without a colloid present in the encapsulation medium to control capsulesize distribution, the above procedure was repeated. After 1 hr thecapsule walls had been dispersed and two liquid phases were present. Theupper layer appeared composed only of free capsule fill liquid.

As will be apparent to those skilled in the art, various othermodifications can be carried out from the above disclosure withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. Microcapsules having a 50% volumetric diameterranging between about 3 to 12 micrometers, said microcapsules comprisingan oleophilic fill material retained within a synthetic thermosetpolymer shell, said shell further comprising colloidal inorganicparticles, said particles having average diameter of less than about0.03 micrometers and having a surface energy selected such that duringmanufacture of the microcapsule from a solution having an oil phase andan aqueous phase, the particle will migrate to the interface of the oilphase and the aqueous phase, wherein said capsule has reducedpermeability such that it exhibits a percent weight loss of less than0.25% after 250 minutes at 100° C.
 2. The microcapsules of claim 1,wherein said colloidal particles are a silica sol.
 3. The microcapsulesof claim 1, wherein said colloidal particles are zirconium dioxide. 4.The microcapsules of claim 1, wherein said oleophilic fill comprises acolor-former dissolved in a hydrophobic solvent.
 5. A sheet materialcoated with the microcapsules of claim
 1. 6. The microcapsules of claim1, wherein said thermoset polymer is an aminoplast polymer.
 7. Themicrocapsules of claim 1, wherein said thermoset polymer isurea-formaldehyde.
 8. The microcapsules of claim 1, wherein saidthermoset polymer is urea-melamine-formaldehyde.
 9. The microcapsules ofclaim 1, wherein said thermoset polymer is selected from the groupconsisting of polyester, polyamide, and polyurea polymers.
 10. Themicrocapsules of claim 1, comprising by weight 9-50% urea formaldehydepolymer (dry weight); 1.5-3.5% of colloidal silica particles (dryweight); and 50-87% of oleophilic fill material.